Thales is credited with Thales' Theorem, which states that in
a circle, the diameter always subtends a right angle from the
circumference.

A generalisation of Thales' Theorem says that the angle
subtended at the centre of a circle by a chord is exactly
double the angle subtended at the circumference. Relatively
easy to prove, it nonetheless forms the basis of a method
of computing one's position from a collection of relative
bearings.